1. Field of the Invention
The present invention relates to a welding method of an aluminum foil, and specifically to a welding method of an aluminum foil used in an aluminum electrolytic capacitor.
2. Description of the Related Art
In aluminum electrolytic capacitors, a chemical conversion coated layer of Al.sub.2 O.sub.3, which serves as a dielectric, is formed on the surface of an aluminum foil. In such an electrolytic capacitor, a lead wire such as a CP wire is connected to the aluminum foil in order to electrically connect the capacitor to the outside.
The connection of the aluminum foil to the lead wire has heretofore been conducted by a method illustrated in FIG. 4. Namely, an aluminum tab 43 provided on a free end of a lead wire 42 such as a CP wire has been connected to an aluminum foil 41 by rivets 44.
As described above, the lead wire has heretofore been connected to the aluminum foil through the aluminum tab. However, it is needless to say that the direct connection of the lead wire to the aluminum foil without using such a tab is preferred, if possible, from the viewpoints of reduction in the number of processes and the number of parts, and the like. However, it has hitherto been impossible for the following reason to directly connect the lead wire to the aluminum foil.
Namely, in order to conduct the resistance welding of a lead wire to an aluminum foil, it is necessary to place an aluminum foil 41 on a lower electrode 51 made of copper, arrange a lead wire 42 on the aluminum foil 41 and then bring an upper electrode tip 52 into contact under pressure with the lead wire 42 at a proper position so as to cause a welding current to flow between the upper electrode tip 52 and the copper-made lower electrode 51 as illustrated in FIG. 5. However, since a chemical conversion coated layer 45 of Al.sub.2 O.sub.3 has been formed to a thickness of about 0.1 .mu.m-10 .mu.m on both sides of the aluminum foil 41, and it has a high electric resistance, it has been impossible to cause the welding current to flow between the upper electrode tip 52 and the copper-made lower electrode 51 under the usual conditions for resistance welding, resulting in a failure to directly weld the aluminum foil 41 to the lead wire 42.